Grey scale contrast in a 3d image display device

ABSTRACT

A display device for displaying a three dimensional image such that different views are displayed according to the viewing angle has a display panel with a plurality of separately addressable pixels for displaying said image. The pixels are grouped such that different pixels in a group correspond to different views of the image. A display driver controls a transmission characteristic of each pixel to generate an image according to received image data. The drive signals applied to each pixel in the display panel are adjusted using grey scale correction values that vary the optical transmission of each pixel within a group so as to produce an image grey scale for each point in the image that is independent of viewing direction.

The present invention relates to display devices, and in particular todisplay devices adapted to display three dimensional or stereoscopicimages.

The generation of three-dimensional images generally requires that adisplay device is capable of providing a different view to the left andthe right eye of a user of the display device. This can be achieved byproviding a separate image directly to each eye of the user by use ofspecially constructed goggles. In one example, a display providesalternating left and right views in a time sequential manner, whichviews are admitted to a corresponding eye of the viewer by synchronisedviewing goggles. In contradistinction, the present invention relates toclasses of display devices where different views of an image can be seenaccording to the viewing angle relative to a single display panel.Hereinafter, these will be referred to generally as 3D display devices.

One known class of such 3D display devices is the liquid crystal displayin which the parallax barrier approach is implemented. Such a system isillustrated in FIG. 1.

With reference to FIG. 1, a display device 100 of the parallax barriertype comprises a back panel 11 that provides a plurality of discretelight sources. As shown, the back panel 11 may be formed by way of anareal light source 12 (such as a photoluminescent panel) covered with anopaque mask or barrier layer 13 having a plurality of slits 14 a to 14 ddistributed across its surface. Each of the slits 14 then acts as a linesource of light.

A liquid crystal display panel (LCD) 15 comprises a plurality of pixels(eg. numbered 1 to 10 in FIG. 1) which are separately addressable byelectrical signals according to known techniques in order to vary theirrespective light transmission characteristics. The back panel 11 isclosely positioned with respect to the LCD panel 15 such that each ofthe line sources 14 of light corresponds to a group 16 of pixels. Forexample, pixels 1 to 5 shown as group 16 ₁ correspond to slit 14 a,pixels 6 to 10 shown as group 16 ₂ correspond to slit 14 b, etc.

Each pixel of a group 16 of pixels corresponds to one view V of aplurality of possible views (V⁻², V⁻¹, V₀, V₁, V₂) of an image such thatthe respective line source 14 a can be viewed through one of the pixels1 to 5 corresponding to that view. The number of pixels in each group 16determines the number of views of an image present, which is five in thearrangement shown. The larger the number of views, the more realisticthe 3D effect becomes and the more oblique viewing angles are provided.

Throughout the present specification, we shall refer to the ‘image’being displayed as the overall image being generated by all pixels inthe display panel, which image is made up of a plurality of ‘views’ asdetermined by the particular viewing angle.

A problem exists with this prior art arrangement. The light transmissioncoefficient of each pixel in the LCD panel is strongly dependent uponthe viewing angle. Thus, if all pixels 1 to 5 are driven equally, theviewed intensity of source 14 a will appear different for differentviews. For example, V₀ will be different than V₂. Thus, the viewedintensity of the source will appear different for different viewingangles.

Conventionally, the perceived intensity of the viewed source for anyparticular element in an image is an important function of properlyrendering grey scales in the image. A conventional display device willprovide drive signals to each pixel of a display panel so as to vary itstransmission coefficient such that a desired grey scale level isachieved for that element of the image. For the 3D display devicedescribed above, if each pixel 1-5 is driven at the same voltage,corresponding to the required grey scale for that element of the image,the resulting grey scale image will be a function of viewing angle. Thisresults in a sub-optimal image and unwanted grey scale artefacts whenobserving the different views of the image.

It is an object of the present invention to overcome or mitigate theunwanted grey scale artefacts in a display device for displaying threedimensional images in which different views of the image are displayedaccording to the viewing angle.

According to one aspect, the present invention provides a display devicefor displaying a three dimensional image such that different views aredisplayed according to the viewing angle, the display device including:

a display panel having a plurality of separately addressable pixels fordisplaying said image, the pixels being grouped such that differentpixels in a group correspond to different views of the image;

a display driver for controlling an optical characteristic of each pixelto generate a grey scale image according to received image data; and

a grey scale compensation device for further controlling said opticalcharacteristic of at least some pixels within a group to compensate fora predetermined viewing angle dependency of said optical characteristic.

According to another aspect, the present invention provides a method fordisplaying a three dimensional image on a display device such thatdifferent views of the image are displayed according to the viewingangle, the method comprising the steps of:

processing image data to form grey scale pixel data values for each oneof a plurality of separately addressable pixels in display panel, thepixels being grouped such that different pixels in a group correspond todifferent views of the image, the pixel data values each for controllingan optical characteristic of a respective pixel to generate a grey scaleimage;

applying grey scale correction values to at least some pixel data valueswithin each group to compensate for a predetermined viewing angledependency of the optical characteristic; and

using the corrected pixel data values to drive pixels of a display panelto generate said image.

Embodiments of the present invention will now be described by way ofexample and with reference to the accompanying drawings in which:

FIG. 1 shows a schematic cross-sectional view of an existing design ofLCD device that uses the parallax barrier approach to display threedimensional images;

FIG. 2 shows a schematic cross-sectional diagram useful in illustratingthe geometry of a parallax barrier LCD device;

FIG. 3 shows a transmission versus voltage curve for a 90 degree twistednematic LCD for viewing angles of φ=0 degrees (ie. normal to the planeof the display) and for φ=50 degrees;

FIG. 4 shows a schematic block diagram of a display device according toembodiments of the present invention;

FIG. 5 shows an embodiment of the invention utilising a lenticulararray;

FIG. 6 shows an alternative form of light source suitable for use withthe display device; and

FIG. 7 shows a graph of viewing angle properties of a conventionalliquid crystal display panel useful in illustrating display optimisationprinciples in accordance with the present invention.

With reference to FIG. 1, the basic function of a parallax barrier type,three dimensional image display device has already been described. Asimilar structure of display panel 15 and back panel 11 illuminationsource may be used in the preferred embodiment of the invention.However, it will be recognised that other configurations may be used aswill become evident hereinafter.

In general, the invention uses a display panel 15 having a plurality ofseparately addressable pixels 1 . . . 10, in which the pixels aregrouped so that the different pixels 1 . . . 5 or 6 . . . 10respectively in a group 16 ₁ and 16 ₂ correspond to different views ofthe image. The display panel 15 may be any suitable electro-opticaldevice in which an optical characteristic of each pixel can be variedaccording to an electrical control signal to generate an image.Preferably the display panel is a liquid crystal display.

An illumination source having a plurality of discrete light sources 14 a. . . 14 d, so that each group 16 of pixels is positioned to receivelight from a respective one of the light sources, is preferablyprovided. This may be by way of the areal light source 12 and mask 13arrangement of FIG. 1, but could also be provided by way of a pixellatedlight source providing light sources 14 as lines of pixels, individualpixels or blocks of pixels.

Still further, the plurality of discrete light sources could be virtuallight sources provided by way of a backlight and lens array (e.g. alenticular sheet array) providing a series of high intensity lightspots. Such an arrangement is illustrated in FIG. 6. A display device 80includes an LCD panel 75, areal light source 72 and a lens array 71. Thelens array focuses light from the areal source 72 into a plurality ofdiscrete focal points 73 just outside the plane of the LCD panel so thateach illuminates a plurality of pixels in the LCD panel, similar to thatdescribed in connection with FIG. 1.

Part of a group of pixels in the display panel 15 is shown in FIG. 2. Alight source 14 of width w corresponds with, and can be viewed through,a group of pixels 0 . . . 7 at respective viewing angles φ₀, φ₁, . . .φ₇ relative to the normal of the plane of the display panel. It will beunderstood that only half of the pixel group 16 is shown, a furtherseven pixels being present to the left of pixel 0 to complete the pixelgroup 16.

Each pixel has a width p₀, p₁, . . . p₇. Preferably, widths p₀ . . . p₇are equal, but they could vary in order to compensate to a certainextent for the angle of incidence of light passing therethrough. Thedistance between the back panel illumination source 14 and the displaypanel 15 is shown as h. In a preferred display device, h=2.3 mm, p₀=200microns, and w=50 microns although these values may be variedsignificantly.

FIG. 3 shows transmission (T) versus voltage (V) characteristics 30 fora display panel 15 in the form of a 90 degree twisted nematic LCD. Thefirst curve 31 (solid line) is the T-V characteristic for a viewingangle φ=0 degrees (e.g. pixel 0). The second curve 32 (broken line) isthe T-V characteristic for a viewing angle φ=50 degrees (e.g. pixel 5).It will be noted that the variation in transmission coefficient for apixel viewed at φ=0 is such that to obtain a suitable grey scale rangean operating voltage of between 0 and V1 is suggested, depending on thegrey scale value required to be displayed. However, it will be notedthat use of the same voltage range to drive pixel 5 will result not onlyin a different set of grey scale values for a given drive voltage, buteven a grey scale inversion in that the slope of the T-V characteristicis reversed.

In accordance with the invention, it is thus proposed to use a differentrange of driving voltages for pixel 5 (φ=50 degrees), namely thatportion lying between V1 and V2, so that the T-V characteristic forpixel 5 is more closely matched to that of pixel 0.

More generally, an appropriate portion of the T-V characteristic may beselected for each viewing angle φ₀ to φ₇ (or for as many angles as arepresent in the display panel).

Yet more generally, compensation may be made for the variations in slopeof the different T-V characteristics of each viewing angle.

Where the T-V characteristics of two different viewing angles aresufficiently close, a common voltage range and/or compensation may bemade for those two viewing angles.

The present invention therefore provides a grey scale compensationdevice that controls the optical characteristic of each pixel 0 . . . 7in a group 16 so as to compensate for the viewing angle.

The grey scale compensation device preferably substantially normalises agrey scale displayed by a group 16 of pixels to that of the other pixelsin the group for any given location in the display panel. The perceivedgrey scale rendering thereby becomes independent of the viewing angle.

Different grey scale correction factors will be required for differentdisplay types and for transmissive versus reflective displays.Appropriate grey scale correction factors can be determined fromappropriately generated transmission/reflection coefficients determinedaccording to techniques known to the person skilled in the art.

FIG. 4 shows schematically exemplary embodiments of a display device 101incorporating a grey scale compensation device.

An image processor 50 receives a stream of image information includinggrey scale pixel data for each of a plurality of views φ₀ . . . φ₇. Theimage information is processed and stored into a frame buffer 51 indigital form so that it can be rendered onto a display device 53. Framebuffer 51 includes a plurality of pages 58, each page including thepixel data for a respective view, φ₀, φ₁, . . . φ₇.

The frame buffer 51 is accessed by a display driver 52 that providesappropriate drive voltage and/or current signals to each pixel of adisplay panel 53 in accordance with each of the stored values in framestore 51. As a general principle, it will be understood that theapplication of grey scale correction values by a grey scale compensationdevice can be applied either:

(i) by digitally modifying the image data stored in the frame store 51to include a correction factor so that the value of drive parameterselected by the display driver 52 is suitably modified, or

(ii) by leaving the image data stored in the frame store 51 unmodified,but applying a correction factor to the output of the display driver 52.

In a first embodiment, a grey scale compensation device 60 (shown indashed outline) is provided as, for example a look-up table accessibleby the image processor 50. The look-up table comprises a plurality ofpages 61, 62, 63 of correction values, each page corresponding to one ofthe viewing angles φ₁ . . . φ₇ to be applied to image data received bythe image processor. The image processor 50 obtains appropriatecorrections to the image data and stores this compensated data in framestore 51.

The expression ‘correction values’ in this context may include‘substitution’ values or ‘offset’ values. In other words, for a giveninput pixel value x_(i), the look-up tables 61-63 may provide asubstitution value x_(s) (as a function of φ) to be stored in the framestore in place of x_(i). Alternatively, for a given input pixel valuex_(i), the look-up tables 61-63 may provide an offset value x_(o) (as afunction of φ) which is combined with the input value and the resultx_(i)+x_(o) stored in the frame store in place of x_(i).

A particular advantage of this embodiment is that it can be implementedwith very little, if any, change in hardware from a conventional LCDdriver arrangement. The functions of the image processor 50 can berealised in software, and the functions of the grey scale compensationdevice 60 can also be realised as a software implementation.

In a variation on this first embodiment, the compensation device 60 mayoperate independently of the image processor 50 upon data already storedin the frame store 51 by the image processor 50. This can be effected byusing a second access port 64 to the frame store 51. The compensationdevice 60 in this embodiment may also be implemented as a softwaremodule, without interfering with the operation of the image processor 50(for example, where this is a customised graphics processor). Again, thelook-up tables 61-63 may provide a substitution value or an offset valueto be implemented by the grey scale compensation device.

In a second embodiment, it is recognised that the grey scalecompensation for each pixel drive signal could be carried out in realtime in the analogue domain, i.e. by applying a correction voltageoffset to each pixel signal produced by the display driver 52. Thus, inthis embodiment, a grey scale compensation device 70 is installedbetween the display driver 52 and the display panel 53 to apply specificoffset voltages and/or currents to those output by the display driver.In this arrangement, the grey scale correction values may be consideredas voltage and/or current offset values.

For the sake of completeness, it is also noted that a hybrid systemcould deploy both techniques of digital correction values applied to theframe store 51 by compensation device 60 and analogue offsets applied tothe display driver outputs by compensation device 70. An appropriatecontribution would be made by both, although this may be a morecomplicated solution. For example, analogue offsets or correction valuesapplied by the grey scale compensation device 70 might be selected tomove the operation of the display panel into an appropriate portion ofthe transmission-voltage characteristic 30, while digital correctionvalues might be selected to compensate for differences in the slope ofthe transmission-voltage characteristics.

It is also noted that the grey scale compensation device 60 as describedherein may also be applied in other forms of 3D display other than thatshown in FIGS. 1 and 2. With reference to FIG. 5, it will be noted thatthe invention can also be applied to a lenticular 3D display device 200.In this lenticular display device, a liquid crystal display panel 115includes a plurality of pixels (a₁ to b₈ are shown) arranged in groups116 ₁, 116 ₂, in similar manner to that in FIG. 1. On top of the LCDarray 115 is positioned a lenticular array 120 of cylindrical lenses121, 122. The lenticular array may include any sheet of corrugatedoptical material, or array of discrete or joined lenses to providelocalised focusing for groups of pixels of the LCD panel.

In the arrangement shown in FIG. 5, the width of each lens element ischosen to be eight pixels, corresponding to an eight-view 3D display. Ofcourse, the width of each lens element may be chosen to correspond todifferent numbers of pixels according to the angular resolutionrequired. The pixels a₁ to a₈ of the LCD are imaged into the differentviews. For example, the light rays emitted from pixels a₂ and a₄ areshown. One sees that in the LCD substrate 116, the rays emitted by pixela₂ propagate to a large extent obliquely with respect to the raysemitted by pixel a₄. The angle between them is, on average,approximately equal to the angle between the two views (θ).

It will be seen that in a lenticular-type 3D display device, the lightrays of the different views travel through the liquid crystal layer atdifferent angles. Therefore, the problem of grey scale dependency of theangle still exists, and is solved by the grey scale compensation device70 as described in connection with FIG. 4.

The invention as described above also has important implications for theoptimisation of liquid crystal displays generally. The viewing angledependence of LCD panels is known generally to be rather poor. FIG. 7illustrates how contrast and grey scale inversion depends upon viewingangle for a standard 90 degree twisted nematic (TN) transmissive LCDwithout compensation foil. The horizontal viewing angle is shown on thex-axis between −60 degrees and +60 degrees from the normal to the planeof the display, and the vertical viewing angle is shown on the y-axisbetween −60 degrees and +60 degrees from the normal to the plane of thedisplay.

The orientations of the optical axes 90, 91 of the LCD polarisers andthe optical axes 92 of the liquid crystal directors are shown in thelower part of the figure.

From FIG. 7, it is seen that the image quality strongly depends uponviewing angle. For the example shown in FIG. 7, the optimal viewingangles are represented by the diagonal line 94 running from top left tobottom right, and grey scale inversion occurs for viewing positions tothe right and above the line 94.

Conventionally, for most important applications such as televisions andcomputer monitors, it is recognised that maximising performance forhorizontal viewing directions is more important than maximisingperformance for vertical viewing directions. For example, for televisionapplications, multiple viewers of a display device will normally bearranged with their eye levels more-or-less consistent relative to thescreen (i.e. with very little variation along the y-axis), but theirhorizontal viewing angles relative to the x-axis may vary significantly.Similarly, a user seated at a computer monitor is more likely to varyhead position along the x-axis while working, than along the y-axis.

According to convention, therefore, the LCD would be rotatedanticlockwise through 45 degrees from the orientation shown in FIG. 7,such that its polarisation axes are at approximately 45 degrees to thex- and y-axes of the display when in use. In this way, the performanceof the display device is optimised for horizontal viewing angles, but iscompromised for vertical viewing angles.

3D LCD displays suffer from the same problems with optimisation ofviewing angle dependency in respect of x and y directions.

However, in the present invention, it is recognised that optimisation ofgrey scale rendering can be achieved by electronic techniques in drivingthe display, using the described grey scale compensation device 60and/or 70 as described above.

Therefore, it is more appropriate to provide the display device with anorientation in which the inherent optical characteristics of the displaypanel are optimised for vertical viewing angle variations. Horizontalviewing angle variations are accommodated for and optimised using theelectronic driving techniques as described herein.

Thus, in a preferred arrangement, the 3D display device described aboveis arranged so that, in normal use, it has the pixels within each group16 that provide different views as a function of angle to a first axisof the display panel, and has the polarising elements of the displaypanel oriented so as to minimise viewing angle dependence relative to asecond axis of the display, where the second axis is orthogonal to thefirst axis.

In a most general sense, the inherent optical characteristics of thedisplay panel are such that viewing angle dependence is reduced orsubstantially minimised relative to the y-axis and the grey scalecompensation device 60 and/or 70 serves to reduce or substantiallyminimise viewing angle dependence relative to an axis that is transverseto the y-axis. More preferably, the grey scale compensation device 60and/or 70 serves to reduce or substantially minimise viewing angledependence relative to an axis that is orthogonal to the y-axis (i.e.the x-axis). In a most preferred device, the x-axis is defined as thehorizontal axis when the display is in normal use, and the y-axis isdefined as the vertical axis when the display is in normal use.

Other embodiments are intentionally within the scope of the accompanyingclaims.

1. A display device (101) for displaying a three dimensional image suchthat different views are displayed according to the viewing angle, thedisplay device including: a display panel (15, 53) having a plurality ofseparately addressable pixels (0 . . . 10) for displaying said image,the pixels being grouped such that different pixels in a group (16)correspond to different views of the image; a display driver (52) forcontrolling an optical characteristic of each pixel to generate a greyscale image according to received image data; and a grey scalecompensation device (60, 70) for further controlling said opticalcharacteristic of at least some pixels within a group to compensate fora predetermined viewing angle dependency of said optical characteristic.2. The display device of claim 1 further including a back panel (11) forproviding a plurality of discrete sources (14) of illumination, eachgroup (16) of pixels in the display panel (15) being positioned toreceive light from a respective one of the discrete sources ofillumination.
 3. The display device of claim 2 in which the back panel(11) provides a plurality of line sources of illumination.
 4. Thedisplay device of claim 2 in which the back panel (11) provides aplurality of point sources of illumination.
 5. The display device ofclaim 2 in which the display panel (15) is a light-transmissive displaypanel adapted for viewing from a side opposite to the side on which theback panel (11) is located.
 6. The display device of claim 1 furtherincluding a lenticular array (120) positioned adjacent to the displaypanel (115), each lenticle (121, 122) within the array focusing lightfrom selected pixels in the display panel.
 7. The display device ofclaim 6 in which each lenticle (121, 122) within the array (120) isassociated with a said group (16) of pixels.
 8. The display device ofclaim 1 in which the optical characteristic is a light transmissioncharacteristic and the display driver (52) and grey scale compensationdevice (60, 70) are adapted to control the amount of light passingthrough each pixel according to a grey scale image to be displayed. 9.The display device of claim 1 in which the grey scale compensationdevice (60) comprises a look-up table containing correction values to beapplied in respect of each pixel within a group.
 10. The display deviceof claim 8 in which the correction values are selected according to theviewing angle of a respective pixel within the group (16).
 11. Thedisplay device of claim 10 in which the correction values are selectedso as to substantially normalise a grey scale intensity displayed by agroup of pixels to be independent of the viewing angle.
 12. The displaydevice of claim 9 in which the look-up table includes substitutionvalues or offset values as a function of viewing angle to be applied toa frame store.
 13. The display device of claim 8 in which the grey scalecompensation device comprises a transmission versus voltagecharacteristic, the grey scale compensation device adapted to adjust apixel drive voltage and/or current received from the display driver. 14.The display device of claim 13 in which the grey scale compensationdevice provides a voltage and/or current offset to the pixel drivevoltage and/or current received from the display driver.
 15. The displaydevice of claim 1 in which the inherent optical characteristics of thedisplay panel (15, 53) are configured such that viewing angle dependenceis reduced or substantially minimised relative to the y-axis and thegrey scale compensation device (60, 70) serves to reduce orsubstantially minimise viewing angle dependence relative to an axis thatis transverse to the y-axis.
 16. The display device of claim 15 in whichthe grey scale compensation device (60, 70) serves to reduce orsubstantially minimise viewing angle dependence relative to an axis thatis orthogonal to the y-axis (i.e. the x-axis).
 17. The display device ofclaim 16 incorporated into an object, in which the x-axis is defined asthe horizontal axis when the object is in normal use, and the y-axis isdefined as the vertical axis when the object is in normal use.
 18. Amethod for displaying a three dimensional image on a display device suchthat different views of the image are displayed according to the viewingangle, the method comprising the steps of: processing image data to formgrey scale pixel data values for each one of a plurality of separatelyaddressable pixels (0 . . . 10) in display panel (15, 53), the pixelsbeing grouped such that different pixels in a group (16) correspond todifferent views of the image, the pixel data values each for controllingan optical characteristic of a respective pixel to generate a grey scaleimage; applying grey scale correction values to at least some pixel datavalues within each group to compensate for a predetermined viewing angledependency of the optical characteristic; and using the corrected pixeldata values to drive pixels of a display panel to generate said image.19. The method of claim 18 in which the optical characteristic is alight transmission characteristic and the grey scale correction valuesapplied are adapted to control the amount of light passing through eachpixel according to a three dimensional grey scale image to be displayed.20. The method of claim 18 in which the grey scale correction values areobtained from a look-up table containing correction values to be appliedin respect of each pixel within a group.
 21. The method of claim 19 inwhich the correction values are selected according to the viewing angleof a respective pixel within the group (16).
 22. The method of claim 21in which the correction values are selected so as to substantiallynormalise a grey scale displayed by a group of pixels to be independentof the viewing angle.
 23. The method of claim 19 in which the grey scalecorrection values are derived from a transmission versus voltagecharacteristic of the display panel, the corrected pixel data valuesbeing used to adjust a pixel drive voltage and/or current applied to thedisplay panel.
 24. The method of claim 18 further including the step ofconfiguring the inherent optical characteristics of the display panel(15, 53) such that viewing angle dependence is reduced or substantiallyminimised relative to the y-axis and applying said grey scale correctionvalues so as to reduce or substantially minimise viewing angledependence relative to an axis that is transverse to the y-axis.
 25. Themethod of claim 24 in which the grey scale correction values are appliedto reduce or substantially minimise viewing angle dependence relative toan axis that is orthogonal to the y-axis (i.e. the x-axis).
 26. Themethod of claim 25 in which the x-axis is the horizontal axis when thedisplay panel is in normal use, and the y-axis is the vertical axis whenthe display panel is in normal use.
 27. A computer program product,comprising a computer readable medium having thereon computer programcode means adapted, when said program is loaded onto a computer, to makethe computer execute the procedure of claim
 18. 28. A computer program,distributable by electronic data transmission, comprising computerprogram code means adapted, when said program is loaded onto a computer,to make the computer execute the procedure of claim 18.